Rotating turbine components can suffer premature failure due to loss of fatigue capability that can result from surface damage caused during manufacturing, assembly or operation. Such surface damage includes manufacturing defects such as forging defects, surface roughness, gouges, notches, tensile residual stresses, metallic inclusions, chemical segregation, oxide defects, etc., and service induced defects such as erosion induced by solid particles or water droplets, corrosion pitting, stress corrosion cracking, foreign object damage, rubbing, fretting, or sliding wear, etc.
A method of enhancing fatigue capability includes application of various types of surface enhancement techniques with various means of applying them such as thermal spray, plating, cladding, physical or chemical vapor deposition, gas or ion nitriding, induction or flame hardening, carburizing, and boriding. Unfortunately, no single technique is effective against all damage mechanisms. Thin coatings lack durability under operating conditions while thick coatings tend to spall off or induce loss of fatigue capability due to parent metal surface damage caused by the coating application process, heat affected zone, coating defects, coating brittleness, chemical alteration of the substrate, thermal expansion mismatch with the parent metal, and/or other incompatibilities caused by the differences in the physical, chemical and/or mechanical characteristics between the coating and the parent metal.
As a result of above concerns there is a need for a dual protection method that will not only provide resistance to the damage mechanisms indicated above, but also provide high tolerance to damage that might occur from the application of a protective layer (e.g., coating) or from the sudden or progressive loss of the protective layer during service.